The present invention relates to a method of unstacking metal sheets.
In industries such as the car industry, in which products are manufactured starting from metal sheets, the said sheets are fed on to the production lines in stacks, from which they are usually unstacked and fed one at a time on to production machines.
Removing a single sheet of the top of the stack is not always an easy job, due to the sheets usually being surface treated with rustproofing oil or grease. At the stacking stage, the air between adjacent sheets is usually expelled by the weight of the sheet/s on top, thus resulting in the formation of a continuous film of lubricating material between the said adjacent sheets. Besides preventing air from entering between the sheets, the said continuous film eventually becomes rubbery, with the result that adjacent sheets are actually glued together.
For assisting the removal of single sheets off a stack, various methods have been proposed, whereby suckers, preferably applied to the corners of the top sheet, are either raised continuously or in oscillating member, so as to separate the top sheet from the one underneath.
According to known methods, separation of adjacent sheets is also assisted by means of air jets, used either singly or in conjunction with the said suckers, and directed crosswise in relation to the edges of the sheets, so as to replace the air expelled between one sheet and the next.
An alternative method consists in the use of toothed blades, which are pressed against the sides of the stack, and then moved upwards so as to hook up and raise the top sheets.
When working with sheets of magnetic material, separation is also known to be assisted using magnets having a transverse magnetic field in relation to the sheets.
The aforementioned methods usually also comprise sensors for detecting whether or not the top sheet has been separated successfully and, if it has not, for repeating the separating cycle. If, after a given number of repeat cycles, the top sheet still fails to be separated successfully, the said sensors emit an emergency signal for arresting the entire production line until the fault is rectified by the operator.
In addition to being unreliable, due to the relative frequency with which the top sheet fails to be separated, a major drawback of the aforementioned known methods is that they fail to prevent stoppage of the production line each time the said top sheet fails to be separated. Such a drawback therefore rules out any possibility of the aforementioned methods being employed on fully automated lines with no supervising personnel.